Negative selection is the major mechanism of central tolerance by which autoreactive T cells are purged from the repertoire. This process depends upon presentation of self-peptides to developing thymocytes by antigen-presenting cells (APCs). Although the question of which APC is responsible for negative selection has been investigated extensively, the role of thymic B cells as APCs in negative selection has been largely neglected. Among bone marrow-derived APCs, which include dendritic cells (DCs), B cells and macrophages, DCs are ascribed to mediate negative selection based on in vitro assays that compare the efficiency of antigen presentation by these different cell populations. Nevertheless, while B cells are poor at presenting antigens via non-specific uptake, they can present antigens that bind to their B cell receptors (BCR) with even higher efficiency than DCs. Our preliminary studies indicate that antigen-specific thymic B cells are excellent APCs for T cell negative selection. Although the presence of B cells in the thymus has been recognized for several decades, their function is not well understood. They are concentrated in the medulla and cortical-medullary junction along with DCs and express high levels of MHC class II. The capacity of thymic B cells to delete thymocytes has been demonstrated in a superantigen model. It remains unclear whether they can mediate negative selection for cognate antigens recognized by BCRs and what their influence on the T cell repertoire is. We have been investigating the role of thymic B cells in negative selection using a system derived from the K/BxN mouse model of inflammatory arthritis, in which both transgene-encoded T cell receptor (TCR) and BCR are specific for the same glucose-6-phosphate isomerase (GPI) self-antigen. Based on our preliminary results, we hypothesize that thymic B cells are efficient APCs in mediating negative selection of T cells for antigens that are recognized by their BCRs and as such contribute to shaping the T cell repertoire. Given the high prevalence of auto- and polyreactive B cell specificities in the immature B cell repertoire, it is likely that B cells within the thymus are capable of presenting a broad range of self-antigens to developing thymocytes. Thus B cell-mediated antigen presentation could represent a novel pathway for negative selection and maintaining T cell tolerance. The insights obtained on the mechanisms of this process may help develop effective antigen- specific therapy for autoimmune diseases. We will pursue the following specific aims: 1) Determine the capacity of antigen-specific thymic B cells in T cell negative selection; 2) Determine the role of thymic B cells in shaping the T cell repertoire; and 3) Determine the contribution of antigen-specific B cells in inducing central versus peripheral T cell tolerance.